The present invention relates to an exhaust gas recirculation (EGR) system for an internal combustion engine for use with vehicles such as automobiles.
In conventional exhaust gas recirculation systems for internal combustion engines for use with vehicles such as automobiles, when the engine is operated in the cold state, exhaust gas recirculation is stopped in order to avoid deterioration of the operating performance of the engine. However, if exhaust gas recirculation is completely stopped it is very difficult to lower the NOx concentration in the exhaust gases, while on the other hand if exhaust gas recirculation is performed in cold operation in the same manner as in warmed-up operation, performance and flexibility of the engine are seriously deteriorated. Therefore, it is contemplated to make a compromise between these conditions and to perform exhaust gas recirculation at a moderate ratio in cold state operation so as not substantially to deteriorate the operating performance and yet substantially to lower the NOx concentration in the exhaust.